Magnetic structures are typically employed in a color CRT for converging the electron beams at the center of the CRT's faceplate. This adjustment is known as statically converging the electron beams, which typically are in an inline array. Each of the magnetic structures is generally comprised of a set of magnetic ring-shaped pole piece pairs, or ring magnets, disposed in a spaced manner along and about the cylindrical neck portion of the CRT. In order to control the convergence of three inline electron beams in a color CRT, three sets of ring-shaped pole piece pairs of magnets are provided, with two-pole, four-pole and six-pole magnets employed in each respective pair of magnetic rings. Static convergence of the electron beams is typically adjusted by means of the four-pole magnet assembly and the six-pole magnet assembly. The four-pole magnet assembly includes two superposed four-pole ring-shaped permanent magnets, while the six-pole magnet assembly includes two six-pole ring-shaped permanent magnets disposed about the CRT's neck portion. An example of this electron beam magnetic convergence arrangement is disclosed in U.S. Pat. No. 3,808,570 to Thompson et al.
The electron beams from the respective electron guns are converged at the center of the fluorescent screen on the CRT's faceplate with the beams undeflected by adjusting the intensity of the magnetic fields produced by the respective magnets of the assembly. Each magnet assembly is typically comprised of two ring-shaped permanent magnets having the same polarity and the same magnetic flux density, or magnetic field strength, which may be rotated to vary the intensity of the magnetic field of the magnet assembly. The intensity of the field of the magnet assembly is minimum when the polarities of the two permanent ring magnets are aligned. The intensity of the field of the magnet assembly may be varied by rotating the permanent magnets relative to each other.
The two ring-shaped permanent magnets are also typically the same size, having the same inner and outer diameters and thickness along the CRT's longitudinal axis. There is an inherent problem with this type of ring-shaped permanent magnet arrangement in that each magnet exerts the same deflection force on the electron beam, causing the electron beam to deviate from its original path giving rise to undesired aberration in the electron gun region or a shift in the beam deflection center resulting in a degradation of video image color purity.
One approach for reducing these undesirable effects is disclosed in U.S. Pat. No. 4,570,140 to Teruaki. In this approach, pairs of ring-shaped permanent magnets having unequal magnetic strength are employed to provide a maximum convergence correction for grossly misaligned beams and a minimum correction when the beams are in substantial convergence. An inlet ring-shaped magnet is provided with a magnetic field strength 10-30% larger than that of the exit ring-shaped magnet. By applying unequal magnetic field strengths to the electron beam along the axis of the CRT, electron beam path deviation is reduced. However, while the two ring-shaped magnets have different magnetic strength, they are virtually identical in other physical characteristics such as size and shape. These two magnets are thus easily interchanged during CRT assembly causing the magnetic field strength on the exit side of the magnet assembly to be greater than that on the entrance side making it very difficult, if not impossible, to realize static convergence of the electron beams on the production line. In addition, providing the two magnets with the desired relative magnetic field strengths increases CRT cost and makes it more difficult to achieve uniformity and quality control in production.
The present invention addresses and resolves the aforementioned problems of the prior art by providing a matched pair of ring-shaped magnetic pole pieces each having the same magnetic field intensity which applies a non-uniform magnetic field to the electron beam for effecting convergence of the electron beams.